Answer:
Complete sentence.
Step-by-step explanation:
A sentence fragment is a clause or a group of words that are part of a sentence but do not form or make a complete sentence. They can be best identified by the structure or composition of the sentence, as it will miss either a subject, a verb or an object, or a complete thought. Even if it contains both a subject and a verb, it can sometimes be a sentence fragment.
The given sentence "the author researched the Great Depression" is a complete sentence as it has all the necessary elements of a complete sentence.